LEDs are widely used in the lighting field. In many lighting applications, it is desirable to utilize many LEDs having the same illumination level or brightness. However, achieving the same, or substantially the same, illumination level for many LEDs can be difficult and may require substantial cost and effort.
Due to practical considerations, e.g., achieving a similar illumination level, LEDs are often connected in series, i.e., in strings having multiple LEDs. Connecting the LEDs of a lighting system in series also allows the lighting system to achieve a supply voltage that can be delivered by a driver power supply unit (PSU) of the lighting system. Even so, in order to avoid exceeding the highest voltage delivered by the PSU, the serially-connected strings of LEDs are often connected in parallel. However, this configuration also has deficiencies.
Notably, the electrical and thermal parameters of LEDs (and LED strings) are not exactly (100%) identical. These differences are created by various factors including, e.g., manufacturing intolerances, inconsistencies in materials, and the like. LEDs are produced by coating a wafer or substrate with various materials through a chemical process, such as epitaxial growth, doping, and the like, to produce a semiconductor material. The semiconductor material is then sliced to create a small die. Wire bonds or other electrical connectors are then added, e.g., by coating or suspension. The assembly is then encapsulated to create a finished LED package.
Inconsistencies in the coating processes and/or dopant materials create significant inherent variations that impact the characteristics of the LEDs including, voltage, lumens, color (temperature) of the LEDs, and the like. Therefore, placing strings of LEDs in parallel does not guarantee that the current will be split equally between the individual strings of LEDs.
Generally, one string of LEDs (i.e., the “hot” channel) will have more current than the other(s), i.e., the “cold” channel(s). In cases of multiple LED strings being connected in parallel, all the strings of LEDs may have different currents and therefore different illumination levels.
Further, the hot channel will produce more heat (even at the same voltage) and will thus have a higher junction (chip) temperature than the “cold” channel(s). The higher temperature chip tends to carry an increasingly higher current (due to the negative temperature coefficient in terms of voltage-ampere (V-A)) which further increases the temperature of the chip. Correspondingly, the cold channel tends to carry increasingly less current and therefore decreases in temperature. This phenomenon renders parallel connected LED circuits unstable, particularly from a current split standpoint.
Because LEDs emit light depending on current, the performance of the unbalanced LEDs will be suboptimal. In some instances, the high temperature LED, i.e., hot channel, can pull current high enough to damage the lighting device. In this scenario, placing the strings of LEDs in parallel will not resolve the problem as the statistical sum of parameters will not eliminate the effect of the negative temperature coefficient in terms of V-A characteristics.
In recognition of the differences in electrical and thermal parameters, LED manufacturers typically provide a datasheet with the LED string that includes the nominal forward voltage (Vf) for producing white light. The nominal forward voltage for white light may be indicated as, for example, Vf=3.2V. However, the datasheet may also indicate that the forward voltage (Vf) has a minimum value, e.g., Vfmin=2.8V, and a maximum value, e.g., Vfmax=3.6V. Due to the variance of the forward voltage, i.e., the difference between Vfmin and Vfmax, the current between two parallel strings of LEDs will not typically be equal. Therefore, the two parallel strings of LEDs will not have the same illumination level or brightness.
In order to address the electrical and thermal differences, particularly the differences that impact light output and color temperature, manufacturers often group or “bin” the LEDs based on lumen, color and voltage. “Binning” allows luminaire manufacturers to select only the LEDs that meet specific and required performance ranges, e.g., voltage ranges. However, binning can be a complex process that groups the LEDs into smaller bins having tighter control of color variation or larger bins having less control of color variation.
More recently, binning has been standardized to specify a bin size that approximately correlates to with the degree of color variation experienced by commercial compact fluorescent lamp (CFL) sources. However, binning adds substantial costs to the LEDs and still fails to ensure that the current is split equally amongst parallel connected strings of LEDs.
In order to split the current between two or more strings of LEDs, other known solutions have utilized a voltage dropping circuit for each LED string. In these systems, the first LED string uses a current generator as a reference for the other LED strings. However, this solution has the disadvantage of the LED strings not being dimmable in pulse width modulation (PWM) or continuous modes.